Page 177 - Origin and Prediction of Abnormal Formation Pressures

d

152 o c dc dr.
W.H. FERTL, G.V. CHILINGAR AND J.O. ROBERTSON JR.
i
I8't~
n
0.9 d~ 9
,,C,.811,0 v 1;5 2. 1. t.5 2.0 1.0 1.,5 2~0 2' 0 .81 1./0 "" 1.5 2.0 ,
"
Illll((lllli) \ .
i i i
-
g
2 lO 6~2 in.

=2 ,._ ...

'
a g 11
1
_
'
10 11111111~111
11,411111 o- I II
A B C D
Fig. 6-1. Plotting the dc-exponent on a logarithmic scale using the mud weight overlay technique in South
Texas (A,D), South Louisiana (B) and Oklahoma (C). (After Zamora, 1972, fig. 2, p. 70. In Chilingarian
and Vorabutr, 1981, fig. 16-5, p. 588.) Note effect of bit size change (from 8 1/2 inches to 6 1/2 inches) on
normal trendline in the South Texas well (D). Trendlines represent constant mud specific weights. (Courtesy
of Oil and Gas Journal.)

lb), and rotary speed (N, in rpm):
log(R/6ON)
d- log(12W/lO6D) (6-1)

Basically, plots of d-exponent versus depth show a decreasing trend with depth. In
transition zones and overpressured environments, in many cases the calculated values
diverge from the normal trend to lower than normal values. Quantitative pressure
evaluation can then be made on using the equivalent depth method (Fertl, 1976, p. 123)
or specially constructed, transparent overlay of parallel, equivalent mud weight lines for
the specific depth scale used for the d-exponent (see Fig. 6-1).
The values of the d-exponent are affected by any change in the basic input parameters
in Eq. 6-1. Furthermore, it is often difficult to establish reasonable values for bit weight
in soft formations. Any major lithologic change in the shale section (e.g., limey or silty
shales, mudstones, and marls) will also affect the d-exponent. The same is true in poorly
maintained drilling fluid systems and for drastic changes in mud weight.
Inasmuch as Eq. 6-1 ignores the direct effect of mud weight on drilling rate, a
modification of the d-exponent has been proposed by Harper (1969) to normalize the
d-exponent for the effective mud weight as follows:
MW1
dc -- d. MW2 (6-2)
where dc is modified (corrected) d-exponent, MW1 is normal mud weight, and MW2 is
actual mud weight used.